High temperature alloy steel

ABSTRACT

An alloy steel suitable for use at high temperatures, having excellent corrosion resistance, creep strength, hot forming characteristics and weldability. The steel is an inexpensive ferrite steel having a composition comprising C up to 0.10 percent, Si up to 1%, MN 0.3 - 1.20 percent, Cr 7-10 percent, Mo 0.3-1.5 percent V 0.1 - 0.3 percent, Nb 0.15 - 0.50 percent, B up to 0.012 percent, N up to 0.050 percent balance fe and incidental impurities.

United States Patent 1 Mimino et al.

[451 Nov. 12,1974

HIGH TEMPERATURE ALLOY STEEL Inventors: Tohru Mimino: KazuhisaKinoshita:

Keisuke Hattori, all of Kawasaki, Japan Assignee: Nippon Kokan KabushikiKaisha,

Kawasaki-shi, Kanagawa-ken, Japan Filed: May 9, 1973 Appl. No.: 358,797

Related US. Application Data Continuation of Ser. No. 63,623, Aug. 13,1970, abandoned.

Foreign Application Priority Data [56] References Cited UNITED STATESPATENTS 3,539,338 11/1970 Mimino 75/126 C 2,109,118 2/1938 Naumann75/126 F 3,044,872 7/1962 Hayes 75/12(\ C 2,572,191 10/1951 Pnyson 1 H75/126 F- 3,290,183 [2/1966 Ohtake 75/l2(1 C 3,201,232 8/1965 Thier75/126 6 Primary E.\aminer-L. Dewayne Rutledge Assistant ExaminerArthurJ. Steiner Attorney, Agent, or FirmFlynn & Frishauf [57] ABSTRACT Analloy steel suitable for use at high temperatures, having excellentcorrosion resistance, creep strength, hot forming characteristics andweldability. The steel is an inexpensive ferrite steel having acomposition comprising C up to 0.10 percent, Si up to 1%, MN 0.3 1.20percent, Cr 7-10 percent, Mo 0.3-1.5 percent V 0.1- 0.3 percent, Nb 0.150.50 percent, B up to 0.012 percent, N up to 0.050 percent balance feand incidental impurities.

6 Claims, N0 Drawings HIGH TEMPERATURE ALLOY STEEL This is acontinuation of application Ser. No. 63,623 filed Aug. 13, 1970, nowabandoned.

U.S. Pat. No. 3,539,338 issued Nov. 10, 1970 discloses related subjectmatter.

This invention relates to a high temperature alloy steel, the alloysteel being an inexpensive ferrite high temperature steel having highcreep rupture strength at temperatures up to about 650C and havingsuperior weldability.

There is a great demand for a high temperature alloy steel tube havinghigh temperature strength and high temperature corrosion resistance atlow cost for use in operations under conditions of high temperature andhigh pressure such as in steam-power units, petroleum refining processesand other petroleum industries.

Heretofore, several kinds of steel tubes have been used according to theconditions of temperature, pressure and atmosphere, but there has-beenproposed no steel tube of suitable characteristics for a temperatureranging from 550C to 650C. The ferrite steel groups of either 2.25% Crl%Mo steel, 5% cr0.5% Mo steel or 9% Cr 1% Mo steel and the austenitestainless steel groups of either 18% Cr 8% Ni or 18% Cr Ni Ti have beenconveniently applied under the above high temperature condition.However, 2.25% Cr 1% Mo ferrite steel does not have a sufficiently hightemperature corrosion resistance to be used at a temperature of overabout 575C and both the 5% Cr 0.5% Mo and 9% Cr 1% Mo ferrite steels areso low in creep rupture strength and so high in quenched hardness in theheat affected portions of a welded structure as to result in cracks.Austenitic stainless steel is very expensive because of the largequantity of expensive Ni addition.

The present invention has been developed on the 2 cent provides a hightemperature corrosion resistance up to 650C similar to that of 9% Cr-1%Mo ferrite alloy steel, and Mo addition of 0.3 to 1.5% Nb addition l of0.15 to 0.50% and V addition of 0.1 to 0.3% form stable carbide andnitride at high temperature to improve high temperature strength. Thishigh temperature strength may be further improved by either individualor combined-addition of 0.001 0.012 percent boron and 0.015 -0.050percent nitrogen in addition to the above listed alloying compositions.

Preferred high temperature alloy steel of the present invention consistsessentially of the following elements in the following weightpercentages:

carbon 0.04 0.06% silicon 0.45 0.60%

manganese 0.50 0.75% chromiun 7.75 8.50% molybdenum 0.9 1.00% vanadium0.15 0.25% niobium 0.30 0.35%

' boron 0.007 0.008% nitrogen 0.01 l 0.030%

' the balance being iron and incidental impurities.

The high temperature alloy steel of the present invention ischaracterized in that the maximum hardness of weld heat affectedportions is so low that there is little possibility for low temperaturecracking while welding. This excellent characteristic is due to the factthat C content is limited to less than 0.10 percent and is stabilizedbecause of the combination of carbon into such carbides as NbC, VC andthe like.

The embodiments of the present invention will be described in detailhereinafter.

The typical chemical compositions of this invention alloy steel is shownin the following Table l.

basis of a careful study and numerous experiments with theafore-mentioned facts, namely the necessity for a high temperature alloysteel manufactured at low cost for use in modern industries and thedefects of the conventionally used ferrite steels, in order to provide aferritic heat resistant alloy steel of excellent mechanical propertiesat low cost. In other words, this invention has made an improvement inhigh temperature strength, hot forming properties, cold formingproperties and weldability of a 9% Cr 1% Mo ferrite alloy steel.

The high temperature alloy steel of this invention transformation pointranging from 820C to 835C,

comprises less than 0.10% carbon, less than 1.0% silicon, 0.30 1.20%manganese, 7-10% chromium 0.3 1.5% molybdenum, 0.1 0.3% vanadium, 0.150.50% niobium and the balance being iron and incidental impurities.

The compositions of the present alloy steel will be described in detailas follows: Cr addition of 7 to l0 perthus enabling tempering at atemperature of approximately 800C. The above listed alloy steels,containing carbides of Nb and V which are effective for creep rupturestrength, easily obtain hardnesses of HV to by 30 minutes heating at atemperature of about 1,115C and tempering at a temperature of about 800Cafter air cooling, thus cold forming such as cold bending can beperformed with ease.

The following Table 2 shows the comparison of the creep rupture strengthbetween the completely annealed materials of this invention alloy steelslisted hereinabove, after heat treatment, and 2.25% Cr- 1% Mo'steel, 5%Cr 0.5% Mo steel and 9% Cr-1% Mo steel, respectively.

Table 2 Creep Rupture Strength (kg/mm Kinds of Steel 650C 10 hrs. 10"hrs. 10 hrs. 10 hrs.

2.25% Crl% M 1.0] 7.3 6.3 4.0 5% Cr0.5% M0 7.3 5.5 4.5 2.8 9% Crl% M08.0 6.0 4.8 3.2 This A 14.1 11.5 9.3 6.2 Invention B 14.1 11.6 9.5 6.4Alloy C 15.0 11.7 9.5 6.3 Steel D 15.0 12.1 10.0 7.1

. Table 3 Maximum Hardness of Weld Heat Affected Portions Kinds of SteelMaximum Hardness (HV) 2.25% Crl% Mo-steel 340 360 5% Cr0.5% M0 steel 410430 9% Crl% Mo steel 440 460 This A 310 lnvention B 280 Alloy C 290Steel D 280 It is apparent from the above Table 3 that the maximumhardness of the alloy steels of this invention ranges from HV 280 to310, being remarkably lower than the maximum hardness of theconventional alloy steels. It is concluded, therefore, that there islittle possibility for low temperature cracking in the steels accordingto the present invention because of martensite transformation during thewelding operation.

The alloy steel of the present invention is useful in fields requiringoperation at temperatures up to 650C and particularly for boilers,petroleum refining processes, general chemical industries, heatexchanger tubes of atomic furnaces and pipe line materials. As has beendiscussed hereinabove, this invention provides a high temperature alloysteel fo the relatively inexpensive ferrite group having high creeprupture strength up to temperatures of about 650C being characterized inthat the maximum hardness of weld heat affected portions is'low andtherefore low temperature cracks are 5 unlikely to be caused whilewelding, thereby proving of great advantage in modern industries.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A high temperature alloy steel having high creep rupture strength upto temperatures of about 650 consisting essentially of the followingelements in the following weight percentages:

carbon less than 0.10%

silicon less than 1.0%

manganese 0.30 1.20% chromium 7 10% molybdenum 0.3 1.5% vanadium 0.10.3% niobium 0.15 0.50% boron up to 0.012% nitrogen up to 0.05%

the balance being iron and incidental impurities.

2. A high temperature alloy steel according to claim 1, containing 0.001to 0.012 percent of boron.

3. A high temperature alloy steel according to claim 1, containing 0.015to 0.050 percent of nitrogen.

4. A high temperature alloy steel according to claim 1, containing 0.001to 0.012 percent of boron and 0.015 to 0.050% of nitrogen.

5. A high temperature alloy steel consisting essentially of thefollowing elements in the following weight percentages.

carbon 0.04 0.06% silicon 0.45 0.60% manganese 0.50 0.75% chromium 7.758.50% molybdenum 0.9 1.00% vanadium 0.15 0.25% niobium 0.30 0.35% boron0.007 0.008%

v nitrogen 0.011 0.030%

650 consisting essentially of the alloy steel of claim 5. 0

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 384'7,6OO Dated November 12, 1974 Inventor(s) TOHRU MIMINO et al It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

On the first page lefthand Column, [30] The priority date "Aug. 8 1969should be --Aug. 27 l969.

Signed and Scaled this thirtieth Day of September 1975 [SEAL] A ttesr:

RUTH. C. MRSON C. MARSHALL DANN Arresting Ojjicer (ummissinnorufl'arents and Trademarkx

1. A HIGH TEMPERATURE ALLOY STEEL HAVING HIGH CREEP RUPTURE STRENGTH UPTO TEMPERATURES OF ABOUT 650* CONSISTING ESSENTIALLY OF THE FOLLOWINGELEMENTS IN THE FOLLOWING WEIGHT PERCENTAGES:
 2. A high temperaturealloy steel according to claim 1, containing 0.001 to 0.012 percent ofboron.
 3. A high temperature alloy steel according to claim 1,containing 0.015 to 0.050 percent of nitrogen.
 4. A high temperaturealloy steel according to claim 1, containing 0.001 to 0.012 percent ofboron and 0.015 to 0.050% of nitrogen.
 5. A HIGH TEMPERATURE ALLOY STEELCONSISTING ESSENTIALLY OF THE FOLLOWING ELEMENTS IN THE FOLLOWING WEIGHTPERCENTAGES.
 6. A high temperature alloy steel tube having high creeprupture strength up to temperatures of about 650* consisting essentiallyof the alloy steel of claim 5.